1. Field of the Invention
The present invention relates to a control of addition and deletion to and from an active set used in a handover control method in a CDMA (Code Division Multiple Access) mobile communications system, and particularly relates to a mobile communications system, a radio network controller and an active set control method for performing active set control in a system in which a high-speed signal is introduced in an uplink radio channel from a mobile terminal to a base station.
2. Description of the Conventional Art
A mobile terminal in a CDMA (Code Division Multiple Access) mobile communications system is capable of simultaneously setting up radio channels between the mobile terminal and a plurality of base stations. In the CDMA mobile communications system, a new radio channel from the mobile terminal to a base station is established before old radio links from the mobile station to other base stations are removed when the mobile station performs handover. The mobile terminal keeps its connection to the other end during the handover operation because the mobile terminal keeps at least one radio link to one of the base stations even when the mobile terminal is performing handover.
A radio network controller (RNC: Radio Network Controller) in the CDMA mobile communications system manages radio channels, which are set between a mobile terminal and each of the base stations during handover, by using active sets of respective mobile terminals. An active set is a list of cells in each of which a mobile terminal sets radio channels (an access link) during handover. The content of the active set varies in accordance with the movement of a mobile terminal during handover. More specifically, a cell included in the active set is deleted or added depending on the location of the mobile terminal which is moving.
The active set is not a list of base stations, but instead, includes a list of cells. The reason thereof is as follows.
If one base station always corresponds to one cell, handover in which the mobile terminal moves from a cell to another cell means handover from a certain base station to another certain base station. However, one base station does not always correspond to one cell. In a case in which one base station forms a plurality of cells with a plurality of directional antennas, the base station corresponds to the plurality of cells. If a cell from which a mobile terminal moves and a cell to which the mobile terminal moves belong to the same base station, then handover between the base stations is not performed even though the mobile terminal performs handover between cells. To deal with such a case, it is necessary that the active set is not a list of base stations to which the mobile terminal sets up radio channels, but instead, includes a list of cells in which the mobile terminals set up radio channels. Additionally, as would be clear to the ordinarily skilled artisan from the above descriptions, the cell included in the list of the active set does not mean an area where effective radio signals reach a base station, but instead, indicates a radio channel which is set up corresponding to the cell between the mobile terminal and the base station.
Descriptions of a conventional handover in the CDMA mobile communications system will be exemplarily described with reference to FIG. 5. FIG. 5 illustrates the CDMA mobile communications system including a radio network controller (RNC: Radio Network Controller) 140 and base stations 120 and 130. Each of the base stations 120 and 130 is connected to the radio network controller 140. The radio network controller 140 is a host apparatus which, for example, sets up radio channels each between a mobile terminal 110 and any of the base stations 120 and 130, and controls handover operations between the base stations 120 and 130. FIG. 5 shows a case where the mobile terminal 110 was in a cell 121 of the base station 120 and moves to a cell 131 of the base station 130.
Descriptions of conventional technologies in operations of active set control, such as the active set selecting method for selecting a cell to be included in the active set, are providing by JP 2000-197092 A, JP 2003-189368 A, and JP 2003-24875 A, which describe conventional methods relating to the operations of active set control, each of which is incorporated herein by reference in their entirety. There are two conventional types of operations for adding/deleting a cell (an access link) to/from the active set. FIGS. 6 and 7 are schematic views respectively for explaining the respective types.
Operations for active set control of a conventional example 1 are for controlling addition/deletion of a cell to/from the active set initiated by the mobile terminal which measures a signal power of a downlink reference channel from each base station. With reference to FIG. 6, detailed descriptions will be provided below of the operations of the conventional example 1, using the case in which the mobile terminal 110 moves from the cell 121 of the base station 120 to the cell 131 of the mobile station 130.
(1) The mobile terminal 110 has a radio channel linked in cell 121. Simultaneously, the mobile terminal 110 periodically measures a downlink reference signal power of the neighboring cell 131.
(2) When the downlink reference signal power of the cell 131 becomes larger, the mobile terminal 110 sends an event trigger (a radio channel setup request signal) for adding the cell 131 to the active set to the radio network controller 140 through the existing radio channel between the mobile terminal 110 and the base station 120.
(3) Upon receiving the event trigger from the mobile terminal 110, the radio network controller 140 sends, to each of the base station 130 and the mobile terminal 110, a radio channel setup signal for setting up a radio channel of the cell 131. The radio network controller 140 also adds the cell 131 to the active set of the mobile terminal 110 in the active set table.
(4) Upon receiving the radio channel setup signal from the radio network controller 140, the base station 130 and the mobile terminal 110 set up a radio channel of cell 131 and start communications on the radio channel of cell 131. Then, the sequence of operations for adding the cell 131 to the active set is completed.
(5) When a downlink signal power of the cell 121 becomes smaller because the mobile terminal has moved from the cell 121 to the cell 131, the mobile terminal 110 sends an event trigger (a radio channel release request signal) for deleting the cell 121 from the active set to the radio network controller 140 through any one of the existing radio channels each between the mobile terminal 110 and any one of the base stations 120 and 130.
(6) Upon receiving the event trigger from the mobile terminal 110, the radio network controller 140 sends, to the base station 120 and the mobile terminal 110, a radio channel release signal for releasing the radio channel of the cell 121. The radio network controller 140 also deletes the cell 121 from the active set of the mobile terminal 110 in the active set table.
(7) Upon receiving the radio channel release signal from the radio network controller 140, the base station 120 and the mobile terminal 110 release the radio channel of the cell 121, between them. Then, the sequence of operations for deleting the cell 121 from the active set is completed.
Operations for active set control of a second way, the conventional example 2 is for controlling a deletion of a cell from the active set initiated by the base station which monitors a receiving condition of an uplink high-speed signal from the mobile terminal. With reference to FIG. 7, detailed descriptions will be provided of the operations of the conventional example 2, using the case where the mobile terminal 110 moves from the cell 121 of base station 120 to the cell 131 of base station 130.
(1) Each of the base stations 120 and 130 respectively forms the cells 121 and 131, which are included in the active set of the radio network controller 140, and measures a received SIR (Signal to Interference power Ratio) of an uplink high-speed signal transmitted from the mobile terminal 110. Each of the base stations 120 and 130 monitors a difference between the received SIR and a target SIR which satisfies the minimum requirement of communications quality for the radio channels and which the radio network controller 140 has provided to the base stations 120 and 130.
(2) When the received SIR becomes lower than a deletion SIR threshold level from the active set (a level determined based on the target SIR) and this state continues for a certain protection time, the base station 120 sends, to the radio network controller 140, a radio channel release request signal for releasing a radio channel of cell 121 (a radio channel of uplink high-speed signal), presuming that the mobile terminal 110 has started sending the uplink high-speed signal in accordance with a rate control signal of the other base station 130 (see FIG. 8).
(3) Upon receiving the radio channel release request signal from the base station 120, the radio network controller 140 sends, to each of the base station 120 and the mobile terminal 110, the radio channel release signal for releasing the radio channel of the cell 121.
(4) Upon receiving the radio channel release signal from the radio network controller 140, each of the base station 120 and the mobile terminal 110 releases the radio channel (the radio channel of uplink high-speed signal) between the mobile terminal 110 and the base station 120. Then, the sequence of operations for deleting the cell 121 from the active set is completed.
In a practical environment of the CDMA communications system, an uplink channel and a down link reference channel (or a rate control signal) have different propagation environments because of a difference in phasing (fluctuations of received signal strength) conditions caused by a difference in frequencies between the uplink high-speed signal and the downlink reference signal (or the rate signal), a difference in interference conditions between the uplink high-speed signal and the downlink reference signal (or the rate control signal), and the like.
For this reason, in the operations in the conventional example 1, it happens that the access link remains in the active set because the downlink channel is satisfying receiving strength for the mobile terminal even if the uplink channel have been degraded and the uplink channel does not work for communications any longer, whereby radio resources are wasted. Effective use of radio resources is strongly required especially under such a coming circumstance that the uplink high-speed channel is used to transmit a large amount of data such as moving images at high speed from the mobile terminal. Since radio resources are not used effectively, in some cases, in the deletion control by the operations of the conventional example 1, the operations of the conventional example 1 are used together with the operations of the conventional example 2.
On the other hand, if the operations of the examples 1 and 2 are used in combination, the operations for addition and deletion of the active set possibly occurs in series in a short time.
As shown in FIG. 8, when the propagation environment is preferable in the downlink radio channel but is not preferable in the uplink radio channel between the mobile terminal 110 and the base station 120, it causes the operations for addition and deletion of the active set in series because of the addition of the cell 121 to the active set in response to the radio channel setup request signal from the mobile terminal 110, and the deletion of the cell 121 from the active set in response to the radio channel release request signal from the base station 120. This results in the deletion and addition occurring in series in a short time.